This invention relates generally to an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst with excellent exhaust purifying capability and durability.
A lean combustion type engine such as a lean bum engine and a direct injection type engine is operated at a lean air-fuel ratio, which is a lower ratio of fuel to air than a stoichiometrical air-fuel ratio, in a predetermined operating range. While the engine is operated at the lean air-fuel ratio, a three-way catalyst cannot satisfactorily purify NOx (nitrogen oxide) in exhaust gases. Accordingly, it is known that the engine is equipped with an NOx catalyst for absorbing NOx in exhaust gases in an oxide atmosphere, and the NOx absorbed by the NOx catalyst is reduced into N2 (nitrogen) in a reduced atmosphere so as to decrease an NOx output into the air. For example, potassium (K) is added as an NOx absorbent agent to the above-mentioned occlusion type lean NOx catalyst in order to improve an NOx absorbing performance as disclosed in Japanese laid-open Patent Publication No. 9-85093.
If, however, the NOx catalyst to which the absorbent agent such as potassium is added is exposed to a high temperature for a long period of time, the catalyst may crack to cause deterioration in the durability of the NOx catalyst.
It is therefore an object of the present invention to provide an exhaust gas purifying catalyst that is able to substantially reduce the deterioration of the exhaust gas purifying performance.
To attain the above object, the present invention provides an exhaust gas purifying catalyst, which includes a carrier and a catalyst layer, and to which at least one material selected from a group of alkali metals and alkali earth metals is added as an absorbent agent. Acid material with a high affinity with respect to the absorbent agent is mixed in the catalyst layer, and an inhibiting layer is formed between the catalyst layer and the carrier so as to inhibit movement of the absorbent agent toward the carrier. Therefore, the acid material fixes the absorbent agent in the catalyst layer, and the inhibiting layer inhibits the movement of the absorbent agent from the catalyst layer toward the carrier. This prevents the evaporation of the absorbent agent and the dissipation of the absorbent agent resulting from its infiltration into the carrier, and also prevents cracking of the catalyst caused by the infiltration of the absorbent agent into the carrier.